There has to be somebody on here who has a answer to this.

Is there really a sigificant difference in how a beachcat floats on saltwater vs freshwater? From a brief google search there is a 2 lb difference in a cubic foot of water. That can't add up to much for how high the boat rides.

I can't say I'd ever noticed, or thought about it. Why do you ask?

As you say, a few percent difference in density, so your boat will float a bit higher in salt water. But I haven't noticed any difference (as a mostly lake sailor).
What I have noticed is the difference in sound. Salt (or sea) water makes more of a shhwshhhing sound (more bubbles?). I thought it was quite noticeable. Anyone else?

I've found salt water tastes nasty , whilst fresh water is much nicer !

never buy a wooden boat that is advertised as fresh water sailed only - fresh water ROTS wood , salt water is a preservative .

when I take my keelboat through crinan canal the difference in bouyancy of the water is obvious on the painted waterline , travellers are also warned their boats float lower than in the sea and to adjust their draft allowances by a couple of inches .

I float in salt water

don't in fresh

salt water can give a nasty "stinging sensation " to a chaffed rear end , whilst fresh can be soothing , especially if peat ladden . (yes racing on lochness can cure your duke of argylls and make your manhood bigger at the same time !)

I think salt water makes a more manly sound like a thwack thump when hitting a wave rather than a feeble thaaarp when encountering a fresh water chop , perhaps its the drugs !!! <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

In windsurfing, you can get on a plane faster in salt as opposed to fresh water due to the board riding higher in the water. I assume the same is true for any boat.

Hobies don't plane they just cut through the water.

With all due respect...It's not fresh or salt water that destroys wood boats but the organisms contained therein. Case in point, the HMS Ontario, a warship from the war of 1812, discovered on the bottom of Lake Erie in very good condition. Masts were still up, ever some glass was still in the windows. The wreck was found in 500 feet of water where fewer organisms live. Google search it, it's pretty cool

John

It's not 2lb per cubic foot - more like 2% to 2.5%. A cubic foot of fresh water weighs about 62.4 lbs (28.3Kilo). A cubic foot of salt water weighs about 63.96lbs (29.0075).

This is significant if you are composed mostly of water (humans) and can make the difference of sink or swim for an egg...but doesn't have much difference on a lightweight catamaran.

Doing some really rough numbers; a cat that weighs 400lbs (sorry, I'm stopping the metric conversions) is going to get about 10 lbs of additional buoyancy in saltwater over fresh. spread out over the waterline length of two 18' hulls (roughly 12" wide at the waterline) will result in the boat floating .053 inches higher in the water...just under 1/16". Add some ripples and you'll never be able to measure the difference.

However, given the error in my approximation of the catamaran hull being square and the quick and dirty numbers, I would say the range would be between 1/16" and 1/8".

Man...you made me do algebra on a Friday...that's cold. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

It was found in Lake Ontario, not Erie. Erie isn't that deep

Thank god we dont sail eggs~!!!

So, since saltwater is denser than freshwater, does that mean that a displacement boat (like most cats) has to work harder to get through it and will therefore be slower in saltwater? (And more likely to pitchpole?)

Whereas, a planing hull on saltwater will ride a little bit higher and get on a plane faster and therefore be faster on saltwater than on freshwater?

Seems like all the boats trying for the 50 mph record are planing boats. Are all the venues used for those speed runs on saltwater?

Surely the salt water displacement is of less volume than fresh water, therefore less wetted surface and less volume of water to displace. So is there a viscosity difference that goes with this that makes salt water slower?

Cheshirecatman

Okay, let's add a lot more salt to the saltwater and make it even thicker. At what point do you say that you are pushing through salt?

Do catamarans sail faster on freshwater or on the Great Salt Lake in Utah?

My bad...you are absolutly correct Lake Ontario

Slight tangent...

I've talked to several surfers that say they can feel differences in the weight of the water due to temp? Cold water being denser of course. Has anyone felt that difference on a boat. You'd really have to be tuned in. Don't forget to calculate temperature in your equation.

Slight tangent of the tangent...

Has anyone ever noticed that 20kts in colder wind feels like more than 20 kts of warmer wind? Case in point 20 kts in Santa Cruz or SF bay feels like it has more power than So Cal.

Densities...Cool.

I think most of this stuff (density, displacement, etc.) isn't noticable by the average person. Maybe at the extremes (like Dead Sea vs. some place with distilled water), or with some gigantic vessel (like an oil tanker)

But I'm not an engineer...

I can't say that I can tell a performance difference in cold vs. warm water (the difference in densities is extremely slight) but I will certainly concur with the differences in the air at different temperatures.

Dry air at 20°C (68°F) is about 1.204 kg/m3.
At 30°C (86°F) it's about 1.165 kg/m3

That's about a 3.35% difference in density - which given everything else is constant, you can certainly feel. Warmer air also holds more moisture, which makes it even less dense. (Water vapor is lighter than air.)

In contrast, the difference between the density of cold water (4°C) vs. warmer water (20°C) is only about 0.18%, which you can almost certainly not feel.

They could have a point. I wasn't going to post this but.....

http://www.lsbu.ac.uk/water/explan5.html#ST

Cheshirecatman

If you look at the air info from an energy perspective, more dense air at the same wind speed = more energy. F=ma or ke = 1/2 mv^2

Where's Timbo when you need him. I'm sure he can tell us about trying to get a plane off the deck on a hot humid day.

I don't know the details of it, but I know it doesn't work as well as on a nice crisp winters day. Heard too many stories from my pilot friends about trying to get fully loaded cesna's, etc off the runway on hot days.

Is it because on the cold day, the motor has more power, or because the wings have more lift?